Friday, August 2, 2013

C++ Parsing Gotchas

C++ is a complicated language. Even just parsing the language is troublesome. Scott Meyer included the most vexing parse as part of his Effective STL book (Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library
). Recently, I encountered another parse issue that ultimately warranted new syntax in recent C++ revisions: the dependent type parse. The idea is simple, type names and value names (i.e., functions or variables) may overlap. That is to say, one might have a type t and a function or variable t in the same namespace. Thus, the parser has to disambiguate. This potential overlap comes up when parsing dependent type names (e.g., set<T>::iterator). Specifically, when declaring a function with a dependent type parameter such as void f(set>T<::iterator I), the compiler will complain that the keyword typename must be inserted in front of the whole dependent type name. Thus, void f(typename set>T<::iterator I) compiles.

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